It is known that an output of an optical rotary encoder, e.g., a pulse signal representing the angular position of an object connected to the rotary encoder, such as a drum, is obtained by at first receiving via a light receiver a light transmitted through a rotating transparent board having a surface coded with a certain number of radially distributed solid (e.g., black) bars of a uniform width at a uniform pitch, and then amplifying the output of the light receiver in a circuit to form a reformed sine or rectangular pulse signal. In a reflection-type linear encoder, a light reflected at a board whose surface is coded in a grating by a reflective color (such as white) and an absorptive color (such as black) is received by a light receiver.
The precision of the output of either encoder depends on the precision of the distributed positions of the transparent (reflective) and solid (absorptive) code bars on the boards. Especially for the rotary encoder, the precision of the output thereof depends on the deflection of the rotation center of the transparent and solid code bars.
The same comments are applicable to magnetic encoders.
Therefore, to obtain an encoder of higher precision, the code boards must be accurately produced and, particularly for a rotary encoder, the rotation center of the code bars must be adjusted in detail, which demands are technically and economically difficult to meet.